- where the z-direction is along the beam pipe and so
- and are the momentum perpendicular to the beam pipe and
- is the (invariant) mass.
Hadron collider physicists use another definition of transverse mass, in the case of a decay into two particles. This is often used when one particle cannot be detected directly but is only indicated by missing transverse energy. In that case, the total energy is unknown and the above definition cannot be used.
- where is the transverse energy of each daughter, a positive quantity defined using its true invariant mass as:
For massless daughters, where , the transverse energy simplifies to , and the transverse mass becomes
- where is the angle between the daughters in the transverse plane:
A distribution of has an end-point at the true mother mass: . This has been used to determine the mass at the Tevatron.